Wholly aromatic polyamide (hereinafter sometimes simply referred to as aramid) fibers include meta-aramid fibers such as Conex® and Nomex® and para-aramid fibers such as Technora®, Kevlar®, and Twaron®.
These aramid fibers have a rigid molecular structure and high crystallinity. Accordingly, as compared with widely used, conventional aliphatic polyamide fibers such as Nylon 6 and Nylon 66, aramid fibers have excellent safety properties in terms of thermal properties such as heat resistance and fire resistance (flame retardancy), chemical resistance, high radiation resistance, electrical characteristics, etc. Therefore, they have been widely used for garment applications such as protective suits that require fire resistance (flame retardancy) and heat resistance, industrial material applications such as bag filters, and interior design applications such as curtains.
Here, methods for obtaining a colored fiber for garment applications and the like generally include a method that utilizes dyeing and a method that utilizes spin-dyeing with dyes and pigments. Further, as a method for coloring a wholly aromatic polyamide fiber, for example, a method that dyes a fiber using a basic dye and a dyeing assistant (carrier) such as benzyl alcohol or acetophenone is generally known.
However, there is a problem in that when wholly aromatic polyamide fibers are dyed with basic dyes such as those used for dyeing aliphatic polyamide fibers, the resulting colored fibers have extremely low light resistance and thus undergo significant fading due to light.
Thus, for the purpose of improving the light resistance of a colored wholly aromatic polyamide fiber, JP-A-49-075824 (Patent Document 1) proposes a method in which an aromatic polyamide solution is dry- or wet-spun, then the obtained fiber is washed, and, before drying, the fiber is impregnated with an aqueous dispersion of a UV-shielding substance. However, this method has a problem in that during carrier dyeing, the carrier is likely to cause the shedding of the UV-shielding substance.
In addition, as a light-resistant wholly aromatic polyamide fiber that can be dyed without a carrier, JP-A-2003-239136 (Patent Document 2) discloses a meta-type wholly aromatic polyamide fiber containing an alkylbenzene sulfonic acid onium salt as a dyeing assistant and a hindered amine light stabilizer. This fiber can be dyed without a carrier, and thus the shedding of a light stabilizer is unlikely to occur during dyeing. However, the addition of the onium salt increases the cost of fiber production. In addition, it decreases the flame retardancy of the resulting fiber, requiring the further addition of a flame retardant or the like.
Further, a light-resistant colored meta-type wholly aromatic polyamide fiber containing a specific pigment that does not fade due to light has been proposed (JP-A-50-59522, Patent Document 3). However, in such a method, the pigment is added during the fiber production process. Accordingly, there are problems in that the loss of production is high, it is difficult to handle small-lot production, it is difficult to obtain fibers in various hues as required, etc.